ECU fix for A/C always on
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Honda-Tech Member
Joined: Apr 2016
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ECU fix for A/C always on
tl;dr Try replacing QM3 (an STA413A transistor) in your ECU if your compressor clutch is always engaged and you’ve isolated problem to the ECU. It’s a commonly failing part.
I know this is an old thread, but hopefully this helps someone else trying to fix this same problem. My AC compressor was always engaged. After much troubleshooting (checking relays, fuses, pressure switch, thermal protector, shorts to ground, etc) I isolated the problem to the ECU always sending ground to the compressor clutch relay, even with the AC switch off. This caused the compressor to run constantly, until it overheated and the thermal protector kicked in... so I’d only get about 15 mins of cold air, then it'd break/cool off for another 15 mins, before the thermal protector closed again and I get cold air. I have a stock ECU in a 93 Civic EX (it’s a P28 with 1720 board)
Basically the problem in the ECU is that one of the transistors is "stuck-closed”. If you open up the ECU and trace back pin A15 (goes to blk/red wire -> ground to compressor clutch relay), it goes to pin 5 on QM3, which is an STA413A transistor array. According to what I’ve read this also controls the fuel pump. This part seems rather unreliable; some forums mention how people replaced this part to fix their fuel problems, so I figure replacing this might fix my AC problem.
Of course, you’ll want to test STA413 first to make sure that this chip is actually bad. Use a multimeter to test for continuity, and look at the online datasheet for "Sanken STA413A" to understand what’s going on inside. Pins are numbered sequentially; pin #1 is at the rounded end of the chip, pin #10 is on the square end. Do these tests on STA413 with the car OFF:
Test 1:
Pins 1 and 10 are ground and should have continuity to each other.
Test 2:
Put the black test probe on Pin 1 (ground). Put red probe on pins 2, 4, 6, 8 - these are the inputs to the 4 transistors, and should have continuity.
Test 3:
Put the black test probe on Pin 1 (ground). Put red probe on pins 3, 5, 7, 9 - these should NOT have continuity.
Honda sprays a clearcoat onto the circuit board, so you might have to scratch some of it off to get a good connection when probing.
In my case, I failed Test 3; my pin 5 was always showing continuity to ground, even with the ECM unplugged and removed from the car. A good chip should pass all three tests.
To get a replacement you can get a new one on ebay for ~$7 shipped, but that takes 2 weeks since they’re mostly overseas sellers. I couldn’t hold my wad so I went to the local junkyard and tried to find the chip there. None of the 92-95 Civics still had their ECU’s (must be popular to resell), but I did manage to find the chip in the ECU of a 92 Accord. If I had to guess, most OBD1 (late 80’s/early 90s) Hondas will have this chip. 96-00 Civics did NOT have it. Anyways I tested the chip using 3 tests above (it passed), then I sawed off the part of the circuit board that I needed, went home and removed/desoldered the rest of the board from the new chip. (I used a handsaw to cut the board and expose the cross section of the pins, then used a screwdriver to pop out the pins, and a soldering iron to remove the remnants of old solder.)
To remove the bad chip from my ECU, first I bent it back and forth until the pins broke from fatigue; then I used a soldering iron and some tweezers to carefully remove the pin remnants. Use a paperclip and soldering iron to make sure the pin holes are clean and clear. Then I pushed in the new chip and soldered it into place.
To make sure I didn’t screw any existing traces up, I tested for continuity between each of the 10 pins and wherever that pin's trace went (check on both sides of the board). Strangely I had a little continuity between pin 3 and ground; not sure if that’s bad or not. Anyway I tied everything back up once I was done, plugged it into the car, and VOILA, car still ran fine, AND AC compressor engages and disengages correctly!
If you’re going to attempt this fix, make sure you have a basic understanding of how electronics work and how to solder. Desoldering the old chip was the hardest part. Last thing you want is to screw up some other part of your board, by breaking a trace, or shorting something with too much solder. (I broke a trace but fixed it with some solder/extra wire.)
I suspect that this same fix would apply for anyone whose compressor NEVER engages because of bad ECU; STA413 could also be "stuck-open" all the time. Maybe plug in ECM and see if it gets +5V on pin 4; if yes and pin 5 still has no continuity to ground, then the chip is probably bad. However I did not test for this case so I can't be sure.
Hope this helps someone else out there!
Pic album (in case embedded links don't work) http://imgur.com/a/1gOfe
Sadly I didn't take a whole lot of pics during the removal/installation process, but here's my board with with some annotations:
Some of the tools and leftovers:
Here's a circuit diagram of the chip in question:
I know this is an old thread, but hopefully this helps someone else trying to fix this same problem. My AC compressor was always engaged. After much troubleshooting (checking relays, fuses, pressure switch, thermal protector, shorts to ground, etc) I isolated the problem to the ECU always sending ground to the compressor clutch relay, even with the AC switch off. This caused the compressor to run constantly, until it overheated and the thermal protector kicked in... so I’d only get about 15 mins of cold air, then it'd break/cool off for another 15 mins, before the thermal protector closed again and I get cold air. I have a stock ECU in a 93 Civic EX (it’s a P28 with 1720 board)
Basically the problem in the ECU is that one of the transistors is "stuck-closed”. If you open up the ECU and trace back pin A15 (goes to blk/red wire -> ground to compressor clutch relay), it goes to pin 5 on QM3, which is an STA413A transistor array. According to what I’ve read this also controls the fuel pump. This part seems rather unreliable; some forums mention how people replaced this part to fix their fuel problems, so I figure replacing this might fix my AC problem.
Of course, you’ll want to test STA413 first to make sure that this chip is actually bad. Use a multimeter to test for continuity, and look at the online datasheet for "Sanken STA413A" to understand what’s going on inside. Pins are numbered sequentially; pin #1 is at the rounded end of the chip, pin #10 is on the square end. Do these tests on STA413 with the car OFF:
Test 1:
Pins 1 and 10 are ground and should have continuity to each other.
Test 2:
Put the black test probe on Pin 1 (ground). Put red probe on pins 2, 4, 6, 8 - these are the inputs to the 4 transistors, and should have continuity.
Test 3:
Put the black test probe on Pin 1 (ground). Put red probe on pins 3, 5, 7, 9 - these should NOT have continuity.
Honda sprays a clearcoat onto the circuit board, so you might have to scratch some of it off to get a good connection when probing.
In my case, I failed Test 3; my pin 5 was always showing continuity to ground, even with the ECM unplugged and removed from the car. A good chip should pass all three tests.
To get a replacement you can get a new one on ebay for ~$7 shipped, but that takes 2 weeks since they’re mostly overseas sellers. I couldn’t hold my wad so I went to the local junkyard and tried to find the chip there. None of the 92-95 Civics still had their ECU’s (must be popular to resell), but I did manage to find the chip in the ECU of a 92 Accord. If I had to guess, most OBD1 (late 80’s/early 90s) Hondas will have this chip. 96-00 Civics did NOT have it. Anyways I tested the chip using 3 tests above (it passed), then I sawed off the part of the circuit board that I needed, went home and removed/desoldered the rest of the board from the new chip. (I used a handsaw to cut the board and expose the cross section of the pins, then used a screwdriver to pop out the pins, and a soldering iron to remove the remnants of old solder.)
To remove the bad chip from my ECU, first I bent it back and forth until the pins broke from fatigue; then I used a soldering iron and some tweezers to carefully remove the pin remnants. Use a paperclip and soldering iron to make sure the pin holes are clean and clear. Then I pushed in the new chip and soldered it into place.
To make sure I didn’t screw any existing traces up, I tested for continuity between each of the 10 pins and wherever that pin's trace went (check on both sides of the board). Strangely I had a little continuity between pin 3 and ground; not sure if that’s bad or not. Anyway I tied everything back up once I was done, plugged it into the car, and VOILA, car still ran fine, AND AC compressor engages and disengages correctly!
If you’re going to attempt this fix, make sure you have a basic understanding of how electronics work and how to solder. Desoldering the old chip was the hardest part. Last thing you want is to screw up some other part of your board, by breaking a trace, or shorting something with too much solder. (I broke a trace but fixed it with some solder/extra wire.)
I suspect that this same fix would apply for anyone whose compressor NEVER engages because of bad ECU; STA413 could also be "stuck-open" all the time. Maybe plug in ECM and see if it gets +5V on pin 4; if yes and pin 5 still has no continuity to ground, then the chip is probably bad. However I did not test for this case so I can't be sure.
Hope this helps someone else out there!
Pic album (in case embedded links don't work) http://imgur.com/a/1gOfe
Sadly I didn't take a whole lot of pics during the removal/installation process, but here's my board with with some annotations:
Some of the tools and leftovers:
Here's a circuit diagram of the chip in question:
Last edited by vader007; Apr 7, 2016 at 10:09 PM. Reason: adding some pics
Trial User
Joined: Nov 2022
Posts: 1
Likes: 0
Re: ECU fix for A/C always on
tl;dr Try replacing QM3 (an STA413A transistor) in your ECU if your compressor clutch is always engaged and you’ve isolated problem to the ECU. It’s a commonly failing part.
I know this is an old thread, but hopefully this helps someone else trying to fix this same problem. My AC compressor was always engaged. After much troubleshooting (checking relays, fuses, pressure switch, thermal protector, shorts to ground, etc) I isolated the problem to the ECU always sending ground to the compressor clutch relay, even with the AC switch off. This caused the compressor to run constantly, until it overheated and the thermal protector kicked in... so I’d only get about 15 mins of cold air, then it'd break/cool off for another 15 mins, before the thermal protector closed again and I get cold air. I have a stock ECU in a 93 Civic EX (it’s a P28 with 1720 board)
Basically the problem in the ECU is that one of the transistors is "stuck-closed”. If you open up the ECU and trace back pin A15 (goes to blk/red wire -> ground to compressor clutch relay), it goes to pin 5 on QM3, which is an STA413A transistor array. According to what I’ve read this also controls the fuel pump. This part seems rather unreliable; some forums mention how people replaced this part to fix their fuel problems, so I figure replacing this might fix my AC problem.
Of course, you’ll want to test STA413 first to make sure that this chip is actually bad. Use a multimeter to test for continuity, and look at the online datasheet for "Sanken STA413A" to understand what’s going on inside. Pins are numbered sequentially; pin #1 is at the rounded end of the chip, pin #10 is on the square end. Do these tests on STA413 with the car OFF:
Test 1:
Pins 1 and 10 are ground and should have continuity to each other.
Test 2:
Put the black test probe on Pin 1 (ground). Put red probe on pins 2, 4, 6, 8 - these are the inputs to the 4 transistors, and should have continuity.
Test 3:
Put the black test probe on Pin 1 (ground). Put red probe on pins 3, 5, 7, 9 - these should NOT have continuity.
Honda sprays a clearcoat onto the circuit board, so you might have to scratch some of it off to get a good connection when probing.
In my case, I failed Test 3; my pin 5 was always showing continuity to ground, even with the ECM unplugged and removed from the car. A good chip should pass all three tests.
To get a replacement you can get a new one on ebay for ~$7 shipped, but that takes 2 weeks since they’re mostly overseas sellers. I couldn’t hold my wad so I went to the local junkyard and tried to find the chip there. None of the 92-95 Civics still had their ECU’s (must be popular to resell), but I did manage to find the chip in the ECU of a 92 Accord. If I had to guess, most OBD1 (late 80’s/early 90s) Hondas will have this chip. 96-00 Civics did NOT have it. Anyways I tested the chip using 3 tests above (it passed), then I sawed off the part of the circuit board that I needed, went home and removed/desoldered the rest of the board from the new chip. (I used a handsaw to cut the board and expose the cross section of the pins, then used a screwdriver to pop out the pins, and a soldering iron to remove the remnants of old solder.)
To remove the bad chip from my ECU, first I bent it back and forth until the pins broke from fatigue; then I used a soldering iron and some tweezers to carefully remove the pin remnants. Use a paperclip and soldering iron to make sure the pin holes are clean and clear. Then I pushed in the new chip and soldered it into place.
To make sure I didn’t screw any existing traces up, I tested for continuity between each of the 10 pins and wherever that pin's trace went (check on both sides of the board). Strangely I had a little continuity between pin 3 and ground; not sure if that’s bad or not. Anyway I tied everything back up once I was done, plugged it into the car, and VOILA, car still ran fine, AND AC compressor engages and disengages correctly!
If you’re going to attempt this fix, make sure you have a basic understanding of how electronics work and how to solder. Desoldering the old chip was the hardest part. Last thing you want is to screw up some other part of your board, by breaking a trace, or shorting something with too much solder. (I broke a trace but fixed it with some solder/extra wire.)
I suspect that this same fix would apply for anyone whose compressor NEVER engages because of bad ECU; STA413 could also be "stuck-open" all the time. Maybe plug in ECM and see if it gets +5V on pin 4; if yes and pin 5 still has no continuity to ground, then the chip is probably bad. However I did not test for this case so I can't be sure.
Hope this helps someone else out there!
Pic album (in case embedded links don't work) http://imgur.com/a/1gOfe
Sadly I didn't take a whole lot of pics during the removal/installation process, but here's my board with with some annotations:
Some of the tools and leftovers:
Here's a circuit diagram of the chip in question:
I know this is an old thread, but hopefully this helps someone else trying to fix this same problem. My AC compressor was always engaged. After much troubleshooting (checking relays, fuses, pressure switch, thermal protector, shorts to ground, etc) I isolated the problem to the ECU always sending ground to the compressor clutch relay, even with the AC switch off. This caused the compressor to run constantly, until it overheated and the thermal protector kicked in... so I’d only get about 15 mins of cold air, then it'd break/cool off for another 15 mins, before the thermal protector closed again and I get cold air. I have a stock ECU in a 93 Civic EX (it’s a P28 with 1720 board)
Basically the problem in the ECU is that one of the transistors is "stuck-closed”. If you open up the ECU and trace back pin A15 (goes to blk/red wire -> ground to compressor clutch relay), it goes to pin 5 on QM3, which is an STA413A transistor array. According to what I’ve read this also controls the fuel pump. This part seems rather unreliable; some forums mention how people replaced this part to fix their fuel problems, so I figure replacing this might fix my AC problem.
Of course, you’ll want to test STA413 first to make sure that this chip is actually bad. Use a multimeter to test for continuity, and look at the online datasheet for "Sanken STA413A" to understand what’s going on inside. Pins are numbered sequentially; pin #1 is at the rounded end of the chip, pin #10 is on the square end. Do these tests on STA413 with the car OFF:
Test 1:
Pins 1 and 10 are ground and should have continuity to each other.
Test 2:
Put the black test probe on Pin 1 (ground). Put red probe on pins 2, 4, 6, 8 - these are the inputs to the 4 transistors, and should have continuity.
Test 3:
Put the black test probe on Pin 1 (ground). Put red probe on pins 3, 5, 7, 9 - these should NOT have continuity.
Honda sprays a clearcoat onto the circuit board, so you might have to scratch some of it off to get a good connection when probing.
In my case, I failed Test 3; my pin 5 was always showing continuity to ground, even with the ECM unplugged and removed from the car. A good chip should pass all three tests.
To get a replacement you can get a new one on ebay for ~$7 shipped, but that takes 2 weeks since they’re mostly overseas sellers. I couldn’t hold my wad so I went to the local junkyard and tried to find the chip there. None of the 92-95 Civics still had their ECU’s (must be popular to resell), but I did manage to find the chip in the ECU of a 92 Accord. If I had to guess, most OBD1 (late 80’s/early 90s) Hondas will have this chip. 96-00 Civics did NOT have it. Anyways I tested the chip using 3 tests above (it passed), then I sawed off the part of the circuit board that I needed, went home and removed/desoldered the rest of the board from the new chip. (I used a handsaw to cut the board and expose the cross section of the pins, then used a screwdriver to pop out the pins, and a soldering iron to remove the remnants of old solder.)
To remove the bad chip from my ECU, first I bent it back and forth until the pins broke from fatigue; then I used a soldering iron and some tweezers to carefully remove the pin remnants. Use a paperclip and soldering iron to make sure the pin holes are clean and clear. Then I pushed in the new chip and soldered it into place.
To make sure I didn’t screw any existing traces up, I tested for continuity between each of the 10 pins and wherever that pin's trace went (check on both sides of the board). Strangely I had a little continuity between pin 3 and ground; not sure if that’s bad or not. Anyway I tied everything back up once I was done, plugged it into the car, and VOILA, car still ran fine, AND AC compressor engages and disengages correctly!
If you’re going to attempt this fix, make sure you have a basic understanding of how electronics work and how to solder. Desoldering the old chip was the hardest part. Last thing you want is to screw up some other part of your board, by breaking a trace, or shorting something with too much solder. (I broke a trace but fixed it with some solder/extra wire.)
I suspect that this same fix would apply for anyone whose compressor NEVER engages because of bad ECU; STA413 could also be "stuck-open" all the time. Maybe plug in ECM and see if it gets +5V on pin 4; if yes and pin 5 still has no continuity to ground, then the chip is probably bad. However I did not test for this case so I can't be sure.
Hope this helps someone else out there!
Pic album (in case embedded links don't work) http://imgur.com/a/1gOfe
Sadly I didn't take a whole lot of pics during the removal/installation process, but here's my board with with some annotations:
Some of the tools and leftovers:
Here's a circuit diagram of the chip in question:
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